Wels catfish

The wels catfish ( or ; Silurus glanis), also called sheatfish or just wels, is a large species of catfish native to wide areas of central, southern, and eastern Europe, in the basins of the Baltic, Black and Caspian Seas. It has been introduced to Western Europe as a prized sport fish and is now found from the United Kingdom east to Kazakhstan and China and south to Greece and Turkey.

Etymology
The English common name comes from Wels, the common name of the species in German language. Wels is a variation of Old High German wal, from Proto-Germanic *hwalaz – the same source as for whale – from Proto-Indo-European *(s)kʷálos ('sheatfish').

Description
The wels catfish's mouth contains lines of numerous small teeth, two long barbels on the upper jaw and four shorter barbels on the lower jaw. It has a long anal fin that extends to the caudal fin, and a small sharp dorsal fin relatively far forward. The wels relies largely on hearing and smell for hunting prey (owing to its sensitive Weberian apparatus and chemoreceptors), although like many other catfish, the species exhibits a tapetum lucidum, providing its eyes with a degree of sensitivity at night, when the species is most active. With its sharp pectoral fins, it creates an eddy to disorient its victim, which the predator sucks into its mouth and swallows whole. The skin is very slimy. Skin colour varies with environment. Clear water will give the fish a black color, while muddy water will often tend to produce green-brown specimens. The underside is always pale yellow to white in colour. Albinistic specimens are known to exist and are caught occasionally. With an elongated body-shape, wels are able to swim backwards like eels.



The female produces up to 30,000 eggs per kilogram of body weight. The male guards the nest until the brood hatches, which, depending on water temperature, can take from three to ten days. If the water level decreases too much or too fast the male has been observed to splash the eggs with its tail in order to keep them wet.

The wels catfish is a long-lived species, with a specimen of 70 years old having been captured during a recent study in Sweden.

Size
With a total length possibly exceeding 3 m and a maximum weight of over 200 kg, the wels is the largest freshwater fish in Europe and Western Asia (only exceeded by the anadromous Atlantic and beluga sturgeon). Such lengths are rare and unproven during the last century, but there is a somewhat credible report from the 19th century of a wels catfish of this size. Brehms Tierleben cites Heckl's and Kner's old reports from the Danube about specimens 3 m long and 200 - 250 kg in weight, and Vogt's 1894 report of a specimen caught in Lake Biel which was 2.2 m long and weighed 68 kg. In 1856, K. T. Kessler wrote about specimens from the Dnieper River which were over 5 m long and weighed up to 400 kg. (According to the Hungarian naturalist Ottó Hermann [1835-1914], catfish of 300–400 kilograms were also caught in Hungary in the old centuries from the Tisza river.)

Most adult wels catfish are about 1.3 - 1.6 m long; fish longer than 2 m are a rarity. At 1.5 m they can weigh 15 - 20 kg and at 2.2 m they can weigh 65 kg.

Only under exceptionally good living circumstances can the wels catfish reach lengths of more than 2 m, as with the record wels catfish of Kiebingen (near Rottenburg, Germany), which was 2.49 m long and weighed 89 kg. Even larger specimens have been caught in Poland (2,61 m. 109 kg.), the former Soviet states (the Dnieper River in Ukraine, the Volga River in Russia and the Ili River in Kazakhstan), France, Spain (in the Ebro), Italy (in the Po and Arno), Serbia (in Gruža Lake, where a 2.4 m long specimen weighing 117 kg was caught on 21 June 2018 and the Danube river, where a catfish measuring 2.75 m and weighing 117 kg was caught at Đerdap gorge in the same year ), and Greece, where this fish was introduced a few decades ago. Greek wels grow well thanks to the mild climate, lack of competition, and good food supply.

The heaviest authenticated specimen, captured from the river Po by a Hungarian fisherman in 2010, weighed 134.97 kg, although there are recent anecdotal reports of larger wels exceeding 300 lb. Meanwhile, the longest wels on record was an unweighed specimen from the Po measuring 2.85 m, captured in 2023 .Exceptionally large specimens are rumored to attack humans in rare instances, a claim investigated by extreme angler Jeremy Wade in an episode of the Animal Planet television series River Monsters following his capture of three fish, two of about 145 lb and one of 164 lb, of which two attempted to attack him following their release. A report in the Austrian newspaper Der Standard on 5 August 2009, mentions a wels catfish dragging a fisherman near Győr, Hungary, under water by his right leg after he attempted to grab the fish in a hold. The man barely escaped from the fish, which he estimated to have weighed over 100 kg.

Diet


Like most freshwater bottom feeders, the wels catfish lives on annelid worms, gastropods, insects, crustaceans and fish. Larger specimens have also been observed to eat frogs, snakes, rats, voles, coypu and aquatic birds such as ducks, even cannibalising on other catfish. Researchers at the University of Toulouse, France, in 2012 documented individuals of this species in an introduced environment lunging out of the water to feed on pigeons at water edge. 28% of the beaching behaviour observed and filmed in this study were successful in bird capture. Stable isotope analyses of catfish stomach contents using carbon-13 and nitrogen-15 revealed a highly variable dietary composition of terrestrial birds. This is likely the result of adapting their behaviour to forage on novel prey in response to new environments upon its introduction to the river Tarn in 1983 since this type of behaviour has not been reported within the native range of this species. They can also eat red worms in the fall, but only the river species.

The wels catfish has also been observed taking advantage of large die-offs of Asian clams to feed on the dead clams at the surface of the water during the daytime. This opportunistic feeding highlights the adaptability of the wels catfish to new food sources, since the species is mainly a nocturnal bottom-feeder.

Distribution and ecology
The wels catfish lives in large, warm lakes and deep, slow-flowing rivers. It prefers to remain in sheltered locations such as holes in the riverbed, sunken trees, etc. It consumes its food in the open water or in the deep, where it can be recognized by its large mouth. Wels catfish are kept in fish ponds as food fish.



An unusual habitat for the species exists inside the Chernobyl exclusion zone, where a small population lives in abandoned cooling ponds and channels at a close distance to the decommissioned power plant. These catfish appear healthy, and are maintaining a position as top predators in the aquatic ecosystem of the immediate area.

As introduced species
There are concerns about the ecological impact of introducing the wels catfish to regions where it is not native. Following the introduction of wels catfish, populations of other fish species have undergone steep declines. Since its introduction in the Mequinenza Reservoir in 1974, it has spread to other parts of the Ebro basin, including its tributaries, especially the Segre River. Some endemic species of Iberian barbels, genus Barbus in the Cyprinidae that were once abundant, especially in the Ebro river, have disappeared due to competition with and predation by wels catfish. The ecology of the river has also changed, with a major growth in aquatic vegetation such as algae.

The wels catfish may have established a population in Santa Catarina, Brazil. They were imported from Hungary in 1988 and were washed into the Itajaí-Açu river after a flood caused their tanks to overflow. In 2006, a specimen weighing 86 kg (189.5 lbs) and 1.85 m (6 ft) long was captured in Blumenau, suggesting the catfish have survived and possibly be reproducing.

Conservation status
Although Silurus glanis is not considered globally endangered, the conservation status varies across the species native distribution range. In the northern periphery of the distribution, the species has been declining over the last centuries and was extinct from Denmark in the 1700s and from Finland in the 1800s. In Sweden it persists only in a few lakes and rivers, and is now considered as near threatened. Recent genetic studies have furthermore found that the Swedish populations harbors low genetic diversity and are genetically isolated and differentiated from each other, highlighting the need for conservation attention.

As food
Only the flesh of young wels catfish specimens is valued as food. It is palatable when the fish weighs less than 15 kg (33 lb). Larger than this size, the fish is highly fatty and additionally can be loaded with toxic contaminants through bioaccumulation due to its position at the top of the food chain. Large specimens are not recommended for consumption, but are sought out as sport fish due to their combativeness.

Wels catfish can be provoked to bite a lure by the sound of a piece of wood plunging into the water, the clonk.

Attacks on people
Tabloids regularly report attacks caused by various catfish that primarily affected animals (often only the role of the catfish was presumed). In April 2009, an Austrian fisherman was allegedly attacked by a catfish in one of the fishing lakes in Pér, near Győr, Hungary. However, the man reportedly managed to break free.

The Wels was the subject of an episode in the first season of the documentary television show River Monsters. Host Jeremy Wade concluded that Wels catfish in the area were not large enough to consume adult human beings, but could easily swallow a child. Wade documented instances of Wels catfish being aggressive towards humans, including a Wels he had just caught that "double[d] round" and attempted to bite his calf.

Similar stories occur in the works of older natural history writers. Alfred Brehm (1829–1884), a German naturalist, published his famous work The World of Animals in the 19th century. It was also translated into Hungarian at the beginning of the 20th century. In this, Brehm or the compiling Hungarian scientists write the following:
 * "Old Gesner’s (Conrad Gessner Swiss naturalist, 1516–1565) claim that catfish doesn’t spare humans either doesn’t just belong in the realm of tales, as we know of several cases that confirm that. Thus, Heckel and Kner mention that a catfish was caught at Bratislava, in the stomach of which the remains of a child corpse were found. [...] Fishermen credible to Antipa (probably Romanian zoologist Grigore Antipa, 1866–1949) told me that children bathing in the stomachs of catfish were caught in the bones of their hands and feet. - Communicates Vutskits (probably Hungarian zoologist hu:Vutskits György, 1858–1929). - A Romanian fisherman penetrated the middle of the Danube with his boat because he wanted to take a bath. While bathing, a catfish caught his legs, which he could no longer pull out of the mouth of this big-mouthed monster, and so he got to the bottom of the water. A few days later, they came across the corpse of a dead fisherman whose legs were still in the catfish's mouth, but even the greedy robber could not release his victim's legs and drowned because of it".

Related species

 * Aristotle's catfish (Silurus aristotelis) from Greece, the only other native European catfish species beside Silurus glanis.
 * Amur catfish (Silurus asotus), introduced to European rivers
 * Giant Lake Biwa catfish (Silurus biwaensis) from Japan endemic to Lake Biwa.
 * Soldatov's catfish (Silurus soldatovi) from the Amur River, Russia